1. Field of the Invention
The present invention relates to titanium oxide-based photocatalysts and self-cleaning materials and the method of preparation thereof. More particularly, the present invention relates to titanium oxide-based photocatalysts having a general formula of TiO2-X-δCXNδ and self-cleaning materials that are prepared by using a process wherein thin films that have a part of oxygen from TiO2 is substituted with C and N by using gases such as N2, CO2, CO are formed, followed by heat treating at a relatively low temperature of about 500° C.
2. Background of the Related Art
Considering the exhaustion of fossil fuels, global warming or environmental problems, the technology for applying sunlight energy that is clean and safe is currently in need. Since Fujishma and Honda have discovered that photocatalysts can be used in decomposition of water and produce hydrogen (A. Fujishima and K. Honda, Nature, 238, 37-38, 1972), there have been many research attempts to use photocatalysts for converting solar energy and to apply them in solar battery. There are a number of materials that can be used for a photocatalyst, however, a representative example is TiO2. Some of the advantages of using TiO2 are that it is chemically stable, harmless to human bodies, and relatively inexpensive compared with other materials.
However, since pure TiO2 photocatalysts can only use ultraviolet rays, the overall efficiency is low, making it difficult to put them into a practical use. Therefore, there has been a need to develop titanium oxide-based photocatalysts that can be activated under the visible light range by lowering the bandgap of TiO2.
On the other hand, irradiation of an ultraviolet ray to the surface of TiO2 makes the surface very hydrophilic. Therefore, the surface does not easily get contaminated with foreign materials, and even if it is contaminated with foreign materials, the foreign materials are decomposed into smaller materials due to the oxidation/decomposition reaction of the photocatalyst. It can be expected that a natural rainfall thereafter would easily cleanse away the materials due to ultra hydrophilic property. Such is so-called the self-cleaning effect and it can be widely used on the surface of glass windows of buildings, glass windows, rear-view mirrors, bodies of automobiles and surfaces of bedpans.
However, these TiO2 photocatalysts, as mentioned above, fall short of their efficiency due to the fact that they can only react at ultraviolet ray range as their energy bandgap is between 3.0 to 3.2 eV. Therefore, in order to also increase the self-cleaning effect, titantium oxide-based photocatalysts that can be activated in the visible light range are also in need.
Many efforts were made to develop titanium oxide-based photocatalysts that exhibit excellent activities under a visible light. As an example thereof include:
1) Method for substituting the Ti part of TiO2 with a transition metal (A. K. Ghosh, H. P. Maruska, J. Electrochem. Soc. Rev. 124, 1516, 197 and W. Choi et al., J. Phys. Chem. 98, 13669, 1994);
2) Method for incorporating oxygen vacancy into TiO2 (R. G. Breckenridge, W. R. Hosler, Phys. Rev. 91, 793, 1953 and D. C. Cronemeyer, Phys. Rev. 113, 1222, 1959);
3) Method for substituting the O part of TiO2 with nitrogen (R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki, Y. Taga, Science 293, 269, 2001); and
4) Method for substituting the O part of TiO2 with carbon (Shahed U. M. Khan, M. Al-Shahry, William B. Ingler Jr. Science 297, 2243, 2002).
When the part of Ti of TiO2 is substituted with a transition metal, although photocatalytic activities such as visible light absorption capability and visible light reactivity can be endowed, the photocatalytic activities are low and the photocatalytic activities of the TiO2 having the original ultraviolet ray range are rapidly reduced, thereby decreasing even more the activities under sunlight illumination. Moreover, the substitution with a transition metal requires high-cost equipment.
For TiO2 with an oxygen vacancy, it has been reported that the reaction efficiency of photocatalysts is significantly increased because the oxygen vacancy produced by lack of oxygen becomes a site for hole electron recombination.
Thin film photocatalysts of pure TiO2 that have a part of oxygen substituted with nitrogen are represented by a general formula of TiO2-XNX. The thin photocatalysts are prepared by first forming thin films on glass plates by having TiO2 target undergo RF Magnetron Sputtering in N2(40%)/Ar combination of gas, then followed by 4 hours of heat treatment at 550° C. under N2 gas again, thereby crystallizing (a mixed phase of anatase and rutile). The thin film photocatalysts having a general formula of TiO2-XNX have an absorption edge moved more than 100 nm towards the ultraviolet A range compared to that of TiO2. As a result, the activities of the thin film photocatalysts having a general formula of TiO2-XNX and pure TiO2 photocatalysts were almost equivalent under ultraviolet irradiation, however, 5 times or more of photocatalytic activities of pure TiO2 photocatalysts were shown for the thin film photocatalysts having a general formula of TiO2-XNX under irradiation of visible light at wavelength of 410 nm or more.
On the other hand, for photocatalysts that have a part of oxygen of TiO2 substituted with carbon and have a general formula of TiO2-XCX, mixed phases of Ti metal rutile and anatase TiO2 were prepared using flame method at a high temperature of 850° C., and it has been reported that the visible light absorption edge of the photocatalysts were moved to 530 nm range.
As discussed above, by having TiO2 substituted with negative or positive ions and thereby increasing the light absorption in the visible light range, many efforts to improve the properties of photocatalysts were made and found that particularly substituting with negative ions was more effective. However, even with the nitrogen substitution, which is known to have the most effective property, not only anatase phase, which has a good photocatalytic property, but also rutile phase, was obtained as a mixture. Furthermore, with the carbon substitution, Ti metal phase in addition to anatase phase and rutile phase also was mixed, and thus, a problem of the low efficiency of the photocatalysts exists.
Therefore, development of a highly efficient titanium oxide-based photocatalysts that can be activated under the visible light and those that comprise only anatase phase is still in need.